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Sustainable Aquaculture: Using Wastewater to Feed Algal Biofuels

In a pioneering 5-month study conducted at a Houston wastewater treatment plant which examined the potential of utilizing municipal wastewater to feed algal-based biofuels, researchers from Rice University discovered that wastewater can be used to produce algae that are rich in oil and thus high in value, while at the same time removing over 90% of nitrates and over 50% of phosphorous — two primary wastewater contaminants — during production.1

"Biofuels were the hot topic in algaculture five years ago, but interest cooled as the algae industry moved toward producing higher-value, lower-volume products for pharmaceuticals, nutritional supplements, cosmetics and other products," said study lead author Meenakshi Bhattacharjee, a member of the Bioscience faculty as Rice University, who has been conducting algal research for 28 years. "The move to high-value products has allowed the algaculture industry to become firmly established, but producers remain heavily dependent on chemical fertilizers. Moving forward, they must address sustainability if they are to progress toward producing higher-volume products, 'green' petrochemical substitutes and fuels."

According to Bhattacharjee, by relying on chemical fertilizers algae growers face two sets of problems: firstly, fertilizers are expensive, and thus eat into profits; and secondly, they find themselves competing for fertilizers with food producers. A report produced by the National Research Council in 2012 revealed that with the technology available, to scale up algal biofuel production to meet just 5% of transportation fuel requirements for the US could increase demands for essential resources such as water, energy and nutrients to unsustainable levels2.

The report suggested that cultivating algae in wastewater could potentially be a viable method of sustainable algae production. This not only offers potential for sustainable algal production, but also offers environmental benefits by reducing nutrient pollutants that would otherwise contaminate freshwater systems.

Chemical fertilizers consist primarily of nitrogen and phosphorous - the nutrients that encourage plant and algal growth. However, these nutrients can leach through soils to contaminate groundwater and drinking water sources, or flow into surface water systems with runoff, where they can cause algal blooms that have a negative impact on freshwater ecology. The US Environmental Protection Agency (EPA) suggests nutrient pollution from fertilizers is "one of America's most widespread, costly and challenging environmental problems."3

Similarly, wastewater is high in nitrates and phosphates, yet wastewater treatment plants are currently unable to remove large volumes of these nutrients during the treatment process in a cost-effective manner. Consequently, utilizing wastewater for algal production could potentially solve both these problems simultaneously as demand for (and use of) fertilizer is reduced, while nutrients present in wastewater are removed by algae and utilized to produce biomass during algal production.

"The idea has been on the books for quite a while, but there are questions, including whether it can be done in open tanks and whether it will be adaptable for monoculture — a preferred process where producers grow one algal strain that's optimized to yield particular products," explained co-author Evan Siemann, a Professor of BioSciences at Rice University. "We were surprised at how little had been done to test these questions. There are a number of laboratory studies, but we found only one previous large-scale study, which was conducted at a wastewater facility in Kansas."

The study, which was a collaboration between a team of Rice University researchers and the Houston Department of Public Works and Engineering, who assisted the team to set up a dozen 600 gallon test tanks at a local wastewater treatment facility, which were subsequently fed filtered wastewater from which suspended solid matter had been removed. The team tested various algal formulations in each of the tanks. Some consisted of monocultures of oil-rich algal species; some consisted of mixed cultures, including local strains found in Houston bayous, while others also contained predatory fish that fed on zooplankton that feed on algae.

"Prior research had suggested that diverse assemblages of algal species might perform better in open tanks and that fish might keep algae-eating zooplankton from adversely affecting yields," said Siemann.

"We recorded prolific algal growth in all 12 tanks," he said. "Our results are likely to be very encouraging to algae producers because the case they would prefer — monocultures with no fish and no cross-contamination — was the case where we saw optimal performance."

According to Bhattacharjee, further studies are needed to ascertain whether wastewater will be a cost-effective solution for algal cultivation, and what circumstances will improve its cost-effectiveness. For example, the researchers found that algae in the latest study conducted at Houston removed phosphorous four times more effectively than algae in the previous study conducted at a wastewater treatment plant in Kansas. Bhattacharjee believes this could be attributed to seasonal and/or regional temperature differences — the former was conducted in the warmer months, with water temperatures in the tanks being approximately 30 degrees warmer than those in the Kansas study.

"Using wastewater would be one of the best solutions to make algaculture sustainable," she said. "If temperature is key, then cultivation may be more economical in the Southeast and Southwest." She noted that other factors, like starting levels of nitrogen and phosphorous, might have caused a rate-limiting effect. "These are the kinds of questions future studies would need to address to optimize this process and make it more attractive for investors," she said.

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